[AMDGPU][llvm-mc] Predefined symbols to access -mcpu from assembly source (.option.machine_...)

[artem.tamazov]

Summary

The feature allows for conditional assembly etc.
It is still unclear how to make those symbols read-only.
Test added.

[vpykhtin]

LGTM, except we should expose the proposed symbol names before submitting, there can be other suggestions.

[SamWot]

lib/Target/AMDGPU/AsmParser/AMDGPUAsmParser.cpp

595

Better place this names to AMDGPUAsmUtils

[artem.tamazov]

In D21238#457366, @vpykhtin wrote:

LGTM, except we should expose the proposed symbol names before submitting, there can be other suggestions.

Suggestions (and subsequent conversations) are good but take time. Let's deliver functionality to our customers without extra delay. We can easy change names to something else, if required. Besides, I've exposed names in the relevant mail thread several days ago, and there is none objections/suggestions))

lib/Target/AMDGPU/AsmParser/AMDGPUAsmParser.cpp

595

Let's see how code will evolve in time. For now, code almost complies with One Definition Rule, so I would better KISS it. Thanks for reviewing.

lib/Target/AMDGPU/AsmParser/AMDGPUAsmParser.cpp

enum KindTy {
     Expression
   } Kind;
 
   SMLoc StartLoc, EndLoc;
 
 public:
   AMDGPUOperand(enum KindTy K) : MCParsedAsmOperand(), Kind(K) {}
 
-  MCContext *Ctx;
-
   typedef std::unique_ptr<AMDGPUOperand> Ptr;
 
   struct Modifiers {
     bool Abs;
     bool Neg;
     bool Sext;
 
     bool hasFPModifiers() const { return Abs || Neg; }
AMDGPUAsmParser(const MCSubtargetInfo &STI, MCAsmParser &_Parser,
     MCAsmParserExtension::Initialize(Parser);
 
     if (getSTI().getFeatureBits().none()) {
       // Set default features.
       copySTI().ToggleFeature("SOUTHERN_ISLANDS");
     }
 
     setAvailableFeatures(ComputeAvailableFeatures(getSTI().getFeatureBits()));
+
+    {
+      // TODO: make those pre-defined variables read-only.
+      // Currently there is none suitable machinery in the core llvm-mc for this.
+      // MCSymbol::isRedefinable is intended for another purpose, and
+      // AsmParser::parseDirectiveSet() cannot be specialized for specific target. 
+      AMDGPU::IsaVersion Isa = AMDGPU::getIsaVersion(getSTI().getFeatureBits());
+      MCContext &Ctx = getContext();
+      MCSymbol *Sym = Ctx.getOrCreateSymbol(Twine(".option.machine_version_major"));
+      Sym->setVariableValue(MCConstantExpr::create(Isa.Major, Ctx));
+      Sym = Ctx.getOrCreateSymbol(Twine(".option.machine_version_minor"));
+      Sym->setVariableValue(MCConstantExpr::create(Isa.Minor, Ctx));
+      Sym = Ctx.getOrCreateSymbol(Twine(".option.machine_version_stepping"));
+      Sym->setVariableValue(MCConstantExpr::create(Isa.Stepping, Ctx));
+    }
   }
 
   AMDGPUTargetStreamer &getTargetStreamer() {
     MCTargetStreamer &TS = *getParser().getStreamer().getTargetStreamer();
     return static_cast<AMDGPUTargetStreamer &>(TS);
   }
 
   void setForcedEncodingSize(unsigned Size) { ForcedEncodingSize = Size; }

test/MC/AMDGPU/symbol_special.s

+// RUN: llvm-mc -arch=amdgcn -mcpu=bonaire %s | FileCheck %s --check-prefix=BONAIRE
+// RUN: llvm-mc -arch=amdgcn -mcpu=hawaii %s | FileCheck %s --check-prefix=HAWAII
+// RUN: llvm-mc -arch=amdgcn -mcpu=tonga %s | FileCheck %s --check-prefix=TONGA
+// RUN: llvm-mc -arch=amdgcn -mcpu=fiji %s | FileCheck %s --check-prefix=FIJI
+
+.if .option.machine_version_major == 0
+.byte 0
+.elseif .option.machine_version_major == 7
+.byte 7
+.elseif .option.machine_version_major == 8
+.byte 8
+.else
+.error "major unknown"
+.endif
+// BONAIRE: .byte 7
+// HAWAII: .byte 7
+// TONGA: .byte 8
+// FIJI: .byte 8
+
+.if .option.machine_version_minor == 0
+.byte 0
+.else
+.error "minor unknown"
+.endif
+// BONAIRE: .byte 0
+// HAWAII: .byte 0
+// TONGA: .byte 0
+// FIJI: .byte 0
+
+.if .option.machine_version_stepping == 0
+.byte 0
+.elseif .option.machine_version_stepping == 1
+.byte 1
+.elseif .option.machine_version_stepping == 3
+.byte 3
+.else
+.error "stepping unknown"
+.endif
+// BONAIRE: .byte 0
+// HAWAII: .byte 1
+// TONGA: .byte 0
+// FIJI: .byte 3
+
+v_add_f32 v0, v0, v[.option.machine_version_major]
+// BONAIRE: v_add_f32_e32 v0, v0, v7
+// HAWAII: v_add_f32_e32 v0, v0, v7
+// TONGA: v_add_f32_e32 v0, v0, v8
+// FIJI: v_add_f32_e32 v0, v0, v8